(1) Field of the Invention
This invention relates to a process for the purification of crude acetonitrile, particularly crude acetonitrile containing impurities such as hydrogen cyanide, allyl alcohol, oxazole and water.
(2) Description of the Prior Art
Acetonitrile of the industrial grade manufactured in the petrochemical industry contains various impurities. For example, crude acetonitrile obtained as a by-product in the manufacture of acrylonitrile or methacrylonitrile by ammoxidation of propylene or isobutylene with oxygen in the presence of a catalyst is usually contaminated with impurities such as hydrogen cyanide, oxazole, water and allyl alcohol, which impurities are considered to be produced as by-products in the ammoxidation process. This crude acetonitrile obtained as a by-product in the manufacture of acrylonitrile or methacrylonitrile by ammoxidation contains acetonitrile at a low concentration and hence, cannot be used as an industrial material. That is, the above-mentioned impurities should be removed from this crude acetonitrile.
Various processes have heretofore been proposed for removing the above-mentioned impurities from crude acetonitrile. For example, there are known a process for removing hydrogen cyanide from crude acetonitrile which comprises converting both hydrogen cyanide and cyanohydrin present in the crude acetonitrile to sodium cyanide by reaction with sodium hydroxide, reacting the resulting sodium cyanide with ferrous sulfate to form sodium ferrocyanide and removing the so formed sodium ferrocyanide (see Japanese Patent Publication No. 35416/72 and Japanese Patent Publication No. 36490/70), and a process for removing hydrogen cyanide from crude acetonitrile which comprises adding an alkali to the crude acetonitrile to adjust the pH of the crude acetonitrile to from 10 to 13.5, and then, treating the crude acetonitrile with formalin in an amount of 0.5 to 2 mols per mol of the total of hydrogen cyanide and cyanohydrin present in the crude acetonitrile (see Japanese Laid-Open Patent Application Specifications No. 81816/73 and No. 25527/75). However, since the former process needs both the alkali treatment and the ferrous sulfate treatment, the steps of the process become complicated and the equipment costs and labor expenses are inevitably increased. Furthermore, since these treatments must be conducted at a high temperature, and already once recovered acetonitrile is readily hydrolyzed and the recovery ratio of acetonitrile is reduced. The latter process involves many steps, and the equipment costs and labor expenses are increased, and if an alkali is added in an amount larger than the equimolar amount to the sum of the hydrogen cyanide and cyanohydrin, decomposition of cyanohydrin takes place and also hydrolysis of acetonitrile is readily caused.
Removal of allyl alcohol contained in crude acetonitrile by rectification is very difficult because the boiling point of allyl alcohol is very close to the boiling point of acetonitrile. As means for removing allyl alcohol from crude acetonitrile, there are known a process comprising adding a hypochlorite and a mineral acid to crude acetonitrile and subjecting the mixture to distillation (see Japanese Patent Publication No. 4531/77), a process comprising adding hydrogen chloride to crude acetonitrile to convert allyl alcohol to allyl chloride and separating the so formed allyl chloride by distillation (see Japanese Laid-Open Patent Application Specification No. 156815/77), and a process comprising adding sulfuric acid to crude acetonitrile and subjecting the reaction mixture to distillation (see Japanese Laid-Open Patent Application Specification No. 23218/76).
In these treatment processes using additive chemicals, since equipments used are readily corroded by these chemicals, it is necessary to use equipment and apparatuses constructed of special materials. Furthermore, special care must be paid to the treatment of waste waters from these processes. Accordingly, it has been eagerly desired in the art to develop a process for removing allyl alcohol from crude acetonitrile by a simple operation without addition of a chemical.
As the process for dehydration purification of crude wet acetonitrile, there are known an extraction dehydration process using calcium chloride and an extraction distillation process using benzene (see U.S. Pat. No. 3,451,899 and Japanese Patent Publication No. 36490/70). However, when organic compound-containing waste water discharged from the extraction step in the former process is treated according to conventional waste water treatment procedures under high temperature and high pressure conditions, the treatment apparatus is readily corroded by chlorine ions present in the waste water, and therefore, special considerations must be taken in the waste water treatment. In the latter process, since water is separated in the form of steam, a large quantity of heat is necessary, and benzene is wastefully consumed because it readily escapes from the extraction distillation system.
Complete removal of oxazole from crude acetonitrile by rectification is very difficult because the boiling point of oxazole is also very close to that of acetonitrile. As means for removing oxazole contained in crude acetonitrile, there is known a process comprising adding a specific inorganic salt to crude acetonitrile to form an oxazole-inorganic salt complex compound, precipitating the complex compound and removing the precipitated complex compound (see British Patent No. 1,156,713). In this process, a special chemical must be used and a very troublesome operation is necessary for the separation of the formed precipitate.
As the process for the purification of crude acetonitrile containing hydrogen cyanide, acrylonitrile and water, U.S. Pat. No. 4,119,497 (patented Oct. 10, 1978) proposes a process comprising adding a base selected from alkali metal hydroxides, ammonium hydroxide, ammonia and aliphatic amines to the crude acetonitrile to convert hydrogen cyanide and acrylonitrile to organic compounds of higher molecular weight, removing water in the form of an azeotropic mixture with benzene in a subsequent azeotropic distillation column and separating the compounds of higher molecular weight in a final rectifying column to recover purified acetonitrile. This process is characterized in that hydrogen cyanide is converted to higher molecular weight compounds according to the following reactions and is removed in the form of these higher molecular weight compounds: ##STR1## The rate of these reactions is slow and thus, it is necessary to use excessive amounts of the bases to complete the above reactions within a reasonably short period of time. Furthermore, when unsaturated nitriles, such as acrylonitrile, are contained as impurities, it is necessary to add the bases in great excess to the crude acetonitrile irrespective of the content of hydrogen cyanide in the crude acetonitrile. This is not preferred because decomposition of acetonitrile is caused.
Hydrogen cyanide contained in crude acetonitrile separated from the reaction product obtained by ammoxidation of propylene or isobutylene is ordinarily present in the form of combined hydrogen cyanide such as cyanohydrin, for example, acetone cyanohydrin. Such cyanohydrin is ordinarily very unstable and separation thereof according to the above process is very difficult. Especially, when the crude acetonitrile contains hydrogen cyanide at a high concentration, a higher temperature and a longer reaction time are necessary for completion of the above chemical reactions, and in this case, decomposition of acetonitrile readily occurs, resulting in reduction of the recovery ratio of acetonitrile.
According to the above process, water is removed by extractive distillation using benzene, and as in the above-mentioned processes proposed in U.S. Pat. No. 3,451,899 and Japanese Patent Publication No. 36490/70, problems of wasteful consumption of benzene arise, owing to the escape of benzene from the distillation system and the mingling of benzene into the purified product. Furthermore, allyl alcohol and oxazole, contained as impurities in crude acetonitrile obtained in the process of ammoxidation of propylene or isobutylene, cannot completely be removed according to this process.